If the server is an esmtp server, you can issue commands link VRFY. There is also another command that you can use. I think it's EXPN or something like that. At any rate, you should be on to a good start. The VRFY command is used to verify that an email address is configured for delivery at said host.
Problem with that is that VRFY and EXPN can both be turned off by the server admin, so all you can really say is that the address doesn't appear to be deliverable if these methods fail.